1. Field of Invention
This invention relates to an improved control valve intended both for positive shut-off and precise characterized flow control of fluid through a valve body.
2. Background
In the valve industry the state-of-the art appears to be such that automated (pneumatically or electrically operated) flow control valves are generally offered with two basic trim (plug and seat) types. One type is the quick opening or "On-Off" trim. The optimum quick opening trim design is one that should produce a maximum fluid flow with minimum plug lift and also provide exceptional shut off (minimal leak rate). Quick opening trim normally operates either in the fully opened (On) position or the fully closed (Off) position. The second type is throttling trim which is readily available in contour forms or "characteristics" such as equal percentage, linear, parabolic, square root, and modified percentage. A trim characteristic is generally displayed graphically as a slope or curve which depicts the relationship of fluid flow volume to plug lift. Characterized trims normally operate at some position between fully open and fully closed.
The quick opening type plugs are frequently manufactured by brazing a metallic ball onto the end of the valve stem whereas the throttling type plugs are usually machined from solid bar material. The radiused ball shape of the quick opening plug serves as both the plug contour (On-Off control) and also the plug seating area. The plug seating area is the actual surface of the plug which contacts the valve seat in the closed position to prevent fluid flow through the valve. In the prior art, characterized plugs have typically been manufactured with various angles resulting in both conically shaped control tips and seating surfaces. The radiused ball type plug which has been used by many valve manufacturers for years is renowned for its inherent feature of self-centering and alignment with the valve seat orifice and superior shut off over the conical plug designs, but its use has been limited solely to the quick opening characteristics to the best of my knowledge.